(537f) Solar Thermal H2o Splitting Via Cobalt Ferrite Based Thermochemical Cycles

Spinel ferrites of the form M_xFe_3-xO₄, where M generally represents Ni, Zn, Co, Mn, Fe or other transition metals, have been shown to be capable of splitting water to produce H₂ using solar energy according to the two step cycle shown below:

M_xFe_3-xO₄ xMO + (3-x)FeO + 0.5 O₂                                                                                                                                                                                 (1)

xMO + (3-x)FeO + H₂O M_xFe_3-xO₄ + H₂                                                                                                                                                                          (2)

The first, high temperature reduction step occurs at temperatures ranging between 1400 ^oC to 1500 ^oC. In the second lower temperature step (~1000 ^oC), the reduced sample is reacted with steam to regenerate the ferrite and produce H₂. Currently, little is understood about the kinetics and mechanisms of these reactions.

We are reporting the results of a kinetic study of the water oxidation (WO) reaction of cobalt ferrites (Co_xFe_3-xO₄), in which the effect of temperature, water concentration, mass loadings and Co/Fe ratio was studied. Ferrites were synthesized via atomic layer deposition (ALD) on high surface area (50 m²/g and 90 m²/g) ZrO₂ monoliths of various surface areas. This technique has the advantage of being able to precisely control the thickness and Co/Fe ratio on the atomic level, resulting in well defined, homogenous samples.

A high temperature stagnation flow reactor was used to carry out both the high temperature reduction step, and the lower temperature water splitting step. Preliminary results indicate that both water concentration, as seen in figure 1, and temperature have a significant impact upon the H₂ reaction rate. Even though water flowrate is 100 times higher than the H₂ reaction rate when flowing only 1% H₂0, the water concentration was rate limiting up to concentrations of 15%. Additionally, the Co/Fe ratio had an effect on the H₂ reaction rate. Fe₃O₄ was characterized by a rapid H₂ increase followed by a rapid decrease. However, CoFe₂O₄ was characterized by a rapid H₂ increase followed by a slow decrease in H₂ over time. A shrinking core model, coupled with O₂ diffusion coefficients in Fe3O4 found in the literature, has been fit to experimental data, as shown in figure 2. Hydrogen evolution was measured in situ using a mass spectrometer, and oxidation state changes were observed in situ via Raman spectroscopy. Surface area was measured via BET analysis, and film composition was determined by ICP-AES and XRD analysis.